1. Field of the Invention
The present invention relates to a failure diagnosis system for an evaporation control system of an automotive vehicle.
2. Description of Related Art
Typically, automobile engines are equipped with evaporation control systems that store fuel tank vapors in a vapor storage canister and supply fuel vapors into a fuel system of the engine. Such an evaporation control system includes a fuel vapor canister for temporary storage of fuel vapors and a purge control valve which is opened to purge fuel vapors in the fuel vapor canister into the fuel system when the engine operates in a specified range of operating conditions. If there is leakage in the evaporation control system between a fuel tank and the vapor storage canister with the purge control valve, it is hard to prevent fuel vapors from escaping into the atmosphere. Therefor, it is typical to make a fuel leakage diagnosis of fuel vapors of the evaporation control system.
A diagnosis of fuel vapor leakage of the evaporation control system is made on the basis of a change in internal pressure of the evaporation control system produced during or after depressurization of the evaporation control system with intake air into the engine which is caused by opening the purge control valve. Such a fuel vapor leakage diagnosis is known from, for example, Japanese Unexamined Patent Publication No. 5-125997 which corresponds to U.S. Pat. No. 5,317,909, entitled xe2x80x9cAbnormality Detecting Apparatus for Use in Fuel Transpiration Prevention Systemxe2x80x9d.
In recent years, there is a strong demand for increasing a chance to implement the diagnosis of failure of the evaporation control system and detecting quite small leakage of fuel vapors. On the other hand, there is a possibility of making a wrong diagnosis because a change in internal pressure of the closed-up evaporation control system that is caused due to small leakage of fuel vapors in the evaporation control system is very small. In particular, a change in the internal pressure of the closed-up evaporation control system is affected by various factors such as the amount of fuel vapors, the temperature of fuel vapors, the amount of remaining liquid fuel, etc. as well as fuel vapor leakage. In light of this, it is thought to limit implementation of the diagnosis of small leakage of fuel vapors to cases where the evaporation control system causes no great change in internal pressure due to factors other than fuel vapor leakage. However, in such a case, a chance to implement the fuel vapor leakage diagnosis considerably diminishes.
It is therefore an object of the present invention to provide a failure diagnosis system for an evaporation control system which makes a fuel vapor leakage diagnosis with a high accuracy while providing a chance to diagnose small leakage of fuel vapors of the evaporation control system increased as much as possible.
In order to achieve the foregoing object of the present invention, according to a preferred embodiment of the present invention, a system for diagnosing failures of an evaporation control system on the basis of a change in internal pressure, negative or positive, of the evaporation control system that is closed up hermetically repeatedly introduces pressure into the evaporation control system and closes up the evaporation control system hermetically and makes a judgement that the evaporation control system causes leakage of fuel vapor on the basis of a plurality of changes in internal pressure of the evaporation control system that occur while the evaporation control system is closed up hermetically
The evaporation control system failure diagnosis system may evade the judgement when a difference between the changes in internal pressure of the evaporation control system is greater than a specified value.
According to another preferred embodiment of the present invention, the evaporation control system failure diagnosis system implements a diagnosis of first type failures such as large leakage of fuel vapors of the evaporation control system in a specified region that is defined with at least engine speed and engine load as parameters when a first diagnostic condition which is determined on the basis of parameters relating to driving conditions is satisfied and a diagnosis of second type failures such as small leakage of fuel vapors of the evaporation control system in the specified region on condition that the vehicle travels in an ordinary driving condition in which changes in the parameters are small when a second diagnostic condition which is determined on the basis of said parameters is satisfied. In this instance, each diagnosis of the first and second type failures is implemented on condition that a specified fuel condition relating to an amount of liquid fuel remaining in a fuel tank is satisfied; and the specified fuel condition is determined so that the diagnosis of second type failures is harder to implement than the diagnosis of first type failures.
According to still another preferred embodiment of the present invention, the evaporation control system failure diagnosis system implements the first diagnosis of first type failures on the basis of only one of the changes in internal pressure of the evaporation control system during engine idling on condition that an amount of fuel vapors generated before introducing pressure into the evaporation control system is small and the diagnosis of second type failures on the basis of a plurality of the changes in internal pressure of the evaporation system during engine off-idling.
Implementing the evaporation control system failure diagnosis on the basis of a plurality of changes in internal pressure of the evaporation control system provides an accurate evaporation control system failure diagnosis as compared with implementation of the evaporation control system failure diagnosis on the basis of a single change in internal pressure of the evaporation control system. Moreover, since implementation of the evaporation control system failure diagnosis is evaded when a difference between changes in internal pressure of the evaporation control system is greater than a specified value on the grounds that there is a possibility of an occurrence of some of such changes in internal pressure of the evaporation control system due to causes other than leakage of fuel vapors, the evaporation control system failure diagnosis is implemented far more accurately. Further, evading implementation of the evaporation control system failure diagnosis eliminates it to provide considerably hard conditions for implementation of the evaporation control system failure diagnosis, which is desirable in order to ensure an increased chance to implement the diagnosis of failure of the evaporation control system.
Separately implementing diagnoses of first and second type failures such as large and small leakage of fuel vapors of the evaporation control system also ensures a chance to implement the diagnosis of failure of the evaporation control system and provides appropriate diagnoses according levels of leakage of fuel vapors. The diagnosis of first type failures, namely large leakage of fuel vapors, is not affected so much by rises in internal pressure of the evaporation control system due to causes other than leakage of fuel vapors, so that wrong diagnoses are avoided and diagnoses are rapid. Further, employing a specified amount of liquid fuel remaining in the fuel tank as one of the conditions for implementation of the evaporation control system failure diagnosis is desirable to avoid wrong diagnoses. In addition, making the specified fuel condition so that the diagnosis of second type failures is harder to implement than the diagnosis of first type failures is desirable to prevent wrong diagnoses of small leakage of fuel vapors.
Implementation of the evaporation control system failure diagnosis during off-idling, i.e. during traveling of the vehicle provides an increased chance to implement the diagnosis of failure of the evaporation control system. Further, implementation of the evaporation control system failure diagnosis both during off-idling and during idling, provides a far more increased chance to implement the diagnosis of failure of the evaporation control system. Further, it is desirable not only in order to perform simply and rapidly diagnoses of failures of the evaporation control system but also in order to perform accurately diagnoses of failures of the evaporation control system to implement the evaporation control system failure diagnosis on the basis of a single change in internal pressure of the evaporation control system during idling and evade it when the amount of fuel vapors generated in the fuel tank is large before introduction of pressure into the fuel tank. Moreover, implementing the evaporation control system failure diagnosis on the basis of a plurality of changes in internal pressure of the evaporation control system during off-idling provides a far more accurate diagnosis as compared with implementation of the evaporation control system failure diagnosis on the basis of a single change in internal pressure of the evaporation control system.